1. Field of the Invention
This invention relates to a monolithic optical memory semiconductor laser apparatus in which a recording semiconductor laser device and a replaying semiconductor laser device are incorporated into a single body.
2. Description of the Prior Art
There are optical disc-filing apparatus, etc., in which as soon as image information is recorded on an optical disc by a recording semiconductor laser device, it is replayed by a replaying semiconductor laser device and if it is recognized that the image information contains errors, a corrected form is rerecorded on the next track of the optical disc. When a recording semiconductor laser device and a replaying semiconductor laser device are monolithically integrated on a single semiconductor substrate, a single optic can be shared for the recording and replaying operations, making the optical disc-filing appparatus extremely simple.
Generally, it is necessary for a recording semiconductor laser device to produce as high an output power as possible. In order to prevent deterioration of the recording semiconductor laser device in the production of a high output power, the light-emitting face of the laser device is covered with a protective film such as alumina (Al.sub.2 O.sub.3) film, etc., having a thickness of (1/4).lambda.(wherein .lambda. is the wavelength of laser light from the laser device).
Since the recording semiconductor laser device and the replaying semiconductor laser device are monolithically formed on a single semiconductor substrate, when the above-mentioned protective film is formed on the recording semiconductor laser device, the formation of the protective film on the replaying semiconductor laser device is unavoidable, which causes reflected light from the optical disc, etc., to strike the replaying semiconductor laser device. Due to such reflected light, the resonance frequency f.sub.0 during the operation of the relaxation oscillation of the replaying semiconductor laser device is reduced as indicated by the equation (1): ##EQU1## wherein fr is the resonance frequency of the relaxation oscillation of the replaying semiconductor laser device and represented by the equation (2): ##EQU2## .tau..sub.s is the life of the carrier, .tau.p is the life of photons, Id is the driving current, Ith is the oscillation threshold current, .tau. is the time required for an optical signal to take a round trip between the semiconductor laser device and the optical disc, C is the light velocity, R.sub.f is the reflection index of the semiconductor laser device, n is the refractive index of the semiconductor laser device, and ld is the cavity length of the semiconductor laser device.
Given that the reflection index F of the above-mentioned optical disc is 0.01 and the reflection index R.sub.f of the replaying semiconductor laser device is 0.02, even though the resonance frequency fr of the replaying semiconductor laser device is several GHz, the replaying semiconductor laser device exhibits a resonance frequency f.sub.0 of as low as about 20-30 MHz, which is close to the frequency zone (from several MHz to several tens of MHz) of the analog signal to be replayed. Therefore, noise intensities in the replaying signal are increased and the replaying signal deteriorates, which causes reading errors which make it difficult to determine as to whether or not the recording semiconductor laser device itself is malfunctioning.
In order to solve the above-mentioned problem, it is conceivable to mask the light-emitting face of the replaying semiconductor laser device when the above-mentioned protective film is formed on the light-emitting face of the recording semiconductor laser device. However, such a process is not practical because extremely precise processing is required which makes production cost significantly high.